Radio relay system



Feb. 7,1939. j l w .L KNoTTs Er A1.

RADIO RELAY SYSTEM Filed Sept. 25, 1936 @imi N5 AI Nl Nl y :Snventor Vl/zrren L. Knot 6 .Beverage Gttorneg Patented Feb. 7, 1939 UNITED STATES RADIO RELAY' SYSTEM Walrr'en L. `Knotts3 (Merchantville, N. J., and Harold H. Beverage, Riverhead, N. Y., assignors to Radio Corporation of Delaware of America, a corporation Application September 25, 1936, Serial No. 102,524

6 Claims.

Our invention relates to ultra high frequency relay systems. More specifically it relates to an ultra high frequency system in which one transmitter and two receivers are used at each relay station and one transmitter and one receiver are used at the terminal stations.

The useful reliable range of an ultra high frequency transmitter is limited to distances of the order of the optical horizon as seen from the transmitting aerial. Relay stations are required for ultra high frequency systems which operate over distances beyond the line of sight from the transmitting antenna to the receiving antenna. It is desirable that such relay stations employ a minimum amount of apparatus which must be so constructed and arranged that undesired feedback and mutual interference effects are avoided.

One of the objects of our invention is to provide means for relaying ultra high frequency waves Without undesirable feedback effects.

Another object is found in the provision of means for relaying with a bidirectional transmitter and receiving with a pair of receivers.

A further object is to provide an`ultra high frequency system for two way communication in which the terminal stations each have one transmitter and one receiver and the relay stations have one bidirectional transmitter and two receivers.

Our invention may be best understood by referring tothe accompanying drawing in which Figure 1 is a schematic illustration of an ultra high frequency system including a pair of relay stations, and

Figure 2 is a circuit diagram of one embodiment of a mixing circuit employed in our invention. Referring to Fig. 1 a terminal station TS1 is comprised of a transmitter T1 and the receiver TR1. The transmitter is comprised of a modulation generator G1 which establishes a modulation frequency F1. This frequency`F1 may be composed of a band of frequencies of the order of 2000 to 4000 cycles per second but since any audio or supersonic frequency may be employed, our invention is not to be limited to any precise frequency, and as hereinafter used the reference characters F1 or F2 designate either a band of frequencies or a single frequency. The output currents from the modulation generator are keyed with the desired signal frequency and impressed on the mixing circuit M1, which is coupled to the ultra high frequency transmitter T1, whose output is radiated by a suitable dipole antenna A1, which may rbe unidirective toward the .rst relay station.

The receiver TR1 of terminal station TS1 is composed of an antenna RA1 and a suitable amplier and detector. The receiver is preferably resonant to only the received signals. The output of the detector circuit of the receiver is imi pressed on aiband pass filter P1 which is designed to pass currents of frequency F2 and reject other frequencies. The output of the filter P1 is coupled to the mixing circuit which also includes a connection to the recorder l.

The relay stations RS2 and RSs are similar; therefore, only a description of one Vstation RS2 will be given. A transmitter T2 is connected to a bidirectional antenna A2 and to a mixing circuit M2. The antenna A2 is directive toward the antenna RA1 and RAS. The mixing circuit M2 is connected to a recorder 2 and a generator G2 which may be used to establish either or both of two dilerent modulation frequencies F1, F2. By way of example, if F1 equals 2000-4000 cycles per second, F2 may be equal to 6000 to 8000 cycles per second. The modulation generator G2 is employed at the relay station only when signals are to be initiated at the relay station.

A pair of radio receivers R2 andV RR2 include dipole antennas RA2 and RRA2, land suitable radio frequency amplifiers and detectors. The receiver R2 is tuned to the waves propagated from A1 and the receiver RR2 is tuned to the frequency of the waves from A3. The output of receiver R2 is connected to a band-pass filter P2 which passes currents of frequency F1 but excludes other frequencies. The receiver RR2 includes a bandpass lter PP2 which passes currents of frequency F2 and excludes other frequencies. The outputs of each of the filters P2 and PP2 are connected to the mixing circuits M2.

Inasmuch as the terminal station TS4 is substantially the same as the first terminal station TS1 no detailed description will be necessary. It will be observed that the modulation frequency 4developed by the generator G4 is F2. The transmission from TS1 toward TSr is effected by modulating the several carriers with frequency F1, while transmission in the direction of vTS4 toTS1 is effected by modulating the carriers with frequency F2. Either of the relay stations RS2 or RSs may initiate signals which are transmitted in either or both directions by employing modulation frequency F1 or F2, or F1 and F2.

The signals to be transmitted may be of any desired type; such as, facsimile, telegraphic or the like. In the schematic illustration the mixing circuits M1, M2, M3 and M1 are designated. Any suitable-.network may vbe employed for 1 vmixing using a plurality of carriers A1, i2, M and M.

purposes. One such network is shown in Fig. 2. In this figure a transformer 5 is comprised of a plurality of windings 6, 1, 8, 9 l0. Each of these windings includes an attenuator and impedance matching network which are, respectively, designed by reference numerals II, l2, I3, I4 and l5.

The outputs from the receiver filters are impressed on a pair of windings 6, 1. The modulator output is impressed on a third winding 8. The recorder is connected to a winding 9, and the transmitter is modulated by a. connection to the remaining winding I0. The attenuators are adjusted to maintain suitable ratios of the several currents.

The operation of the complete system is as follows: The transmitter TS1 transmits a carrier )11 which is modulated by signaling currents at frequency F1. The modulated carrier M is received at RA2, amplified, detected and filtered by filter P2. The signal currents from P2 are impressedon the mixing circuit M2. The generator G2 is idle. Thus the filtered signal current F1 modulates the transmitter T2 and causes a new modulated carrier A2 to be propagated. The modulated carrier is propagated to RA1 and RAS. The carrier )12 will be received by RA1 but the filter P1 will not pass the modulation currents F1. The carrier A2 impressed on the antenna RA2 and receiver R3 will be amplified, detected, passed by the filter P3, and impressed on the mixing circuit M3. The generator G3 is idle. 'I'he signal currents F1 in the mixing circuit M3 modulate T3 which impresses modulated carrier currents M on the antenna A3.

These currents )13 are transmitted to the receiving antennas RRA2 and RA4. The currents )13 modulated at F1 have no effect on the recorder 2 because the filter PP2 does not pass currents of frequency F1. The modulated carrier k3 received by R4 is suitably amplified, detected and passed by filter P4 to the mixing circuit M4 where the signaling currents are induced in the recorder circuit 4. The received signals impressed on M4 will modulate the transmitter, if it is in operation, but such modulation will not affect the relay receiving system RRAa, RRa and PPs because the filter PPs does not pass currents of frequency F1.

The signaling currents F2 are transmitted from terminal station TS4 to terminal station TS1 in the same manner as the transmission from TS1 to 'PS4 just described. It should be understood that the recorders may be facsimile printers, loudspeakers, telephone receivers or any signal indicator. The recorders at the relay stations may be operated as monitors or for normal recording. As previously explained the relay stations, instead of being used as relays, may be used to initiate signaling currents in either or both directions by modulating respectively with currents of frequency F1 or F2 or both F1 and F2.

Thus we have described a system in which undesirable interference effects are avoided by The relay stations are greatly simplified by using modulation frequencies F1 for transmission in one direction and F2 for transmission inthe other. Filters in the several receivers prevent undesired signal modulation currents from interfering with desired signals in the several receivers. The use of mixing circuits makes for simplification of each station and makes it practical to use a common recorder as a monitor. While a bidirectional system has been illustrated,

it should be understood that our invention may be applied to transmission in a plurality of directions by the use of antenna systems which have appropriate field patterns. Likewise a single receiving antenna may be used in place of the pair of dipoles shown.

While we have shown simple dipole antennas for both transmitting and receiving, it should be understood that directive antennas can be, and probably would be, used in most cases. A high degree of directivity may be used if the relay stations are erected in a fairly straight line.

It should be further understood that the same radio frequencies may be repeated in the network at points sufficiently removed since the ultra high frequencies tend to be attenuated rapidly beyond the horizon.

We claim as our invention:

1. A radio relay station comprising a pair of receivers each resonant to different radio frequencies, a first band pass filter connected to the output of the first of said receivers, said first fllter passing only demodulated currents in a first band of frequencies, a second band pass filter connected to the output of the second of said receivers, said second lter passing demodulated currents in a second distinct band of frequencies, a mixing circuit coupled to said filters, a transmitter for generating a radio frequency carrier Whose frequency differs from the previously mentioned radio frequencies, means coupling said mixing circuit to said transmitter whereby signal currents impressed on the mixing circuit will modulate the transmitter.

2. The method of relaying radio signals in a two way communication system which includes the steps of propagating a first signal bearing carrier from a first terminal station in one direction over the system, modulating said signal bearing carrier in a first band of modulation frequencies, receiving said carrier at a relay station, demodulating said modulated carrier to derive demodulated currents in said first band of frequencies, passing said first band of demodulation currents, excluding currents of other frequencies, generating a local carrier of other than said first carrier frequency, impressing said first band of demodulation currents on a mixing circuit and hence on said local signal bearing carrier, receiving said last mentioned signal bearing carrier at a second terminal station, demodulating said carrier, passing currents bearing said first band of demodulated currents, excluding currents of other frequencies, recording said demodulation currents flowing through said mixing circuit, relaying in a similar manner a second signal bearing carrier in the other direction over the system by modulating said second signal bearing carrier in a second band of modulation frequencies, receiving, demodulating, and filtering said signal bearing carrier to pass on the return path only modulation frequencies in said second band of modulation frequencies.

3. In a high frequency two way radio relay system, the combination of a first and a second terminal station and one or more intermediate relay stations, said first terminal station comprising a high frequency transmitter tuned to a first radio frequency, a source of modulation currents falling within a first band of modulation frequencies, a high frequency receiver tuned to a second radio frequency, a band pass filter coupled to the output of said receiver, said filter passing a second band of modulation fretually couple said transmitter, filter, source of modulation currents, and recorder, whereby said transmitter may be modulated and said recorder operated by said first or second band of modulation frequencies, each intermediate relay station comprising a high frequency radio transmitter tuned to a frequency different from that of every other transmitter in the system within its effective range, but equal to the resonant frequency of a receiver at each adjacent station, a pair of high frequency receivers, the first tuned to the frequency of the transmitter next adjacent in the direction of said rst terminal station, and the second tuned to the frequency of the transmitter next adjacent in the direction of said second terminal station, a first band pass filter passing said iirst band of modulation frequencies coupled to the output of said first receiver, a second band pass filter passing said second band of modulation frequencies coupled to the output of said second receiver, a mixing circuit to couple said transmitter to said first and second band pass filters whereby said transmitter may be modulated by currents from said filters, said second terminal station comprising a high frequency transmitter tuned to a fre quency different from that of every other transmitter in the system within its effective range, but equal to the resonant frequency of the receiver at the adjacent relay station which is coupled to the filter passing said second band of modulation frequencies, a source of modulation currents falling within said second band of modulation frequencies, a high frequency receiver resonant to the frequency of the transmitter of the adjacent relay station, a band pass filter coupled to the output of said receiver for passing said first band of modulation frequencies, a recorder operable by said first and second bands of modulation frequencies, and a mixing circuit for mutually coupling said transmitter, filter, source of modulation frequencies, and recorder, whereby said transmitter may be modulated and said recorder operated by said first or second band of modulation frequencies.

4. In an ultra high frequency two Way radio communication system having terminal stations and an intermediate relay station, the method of preventing feed back between transmitters and receivers at the same or succeeding stations, which includes the steps of generating at each of the several stations carrier frequency waves having a frequency different from all other carrier frequency waves in the system, which are within effective reception range, modulating the carrier wave from oneV terminal station in a first band of modulation frequencies, modulating the carrier waves from the other terminal station in a second and distinct band of modulating frequencies, independently receiving and demodulating at each relay station the carrier waves from adjoining stations, to obtain signal currents of modulation frequency, modulating the radio frequency carrier waves at each relay station by said signal currents, filtering and selecting said signal currents at each station so that only modulation frequencies in said first band are relayed in a direction toward said second -terminal station, and so that only modulation frequencies in said second band are relayed in a direction toward said first terminal station.

5. In a two Way radio frequency communication system having an intermediate repeater station actuated by signals received from terminal stations, the method of preventing feed back between transmitters and receivers at the same or adjacent stations which includes the steps of generating local carrier waves at each station, said waves having respectively different frequencies from all other carrier waves in the system within effective range, propagating said carrier waves modulated by signals Within a first band of modulation frequencies from one terminal station, propagating carrier Waves modulated by signals within a distinct second band of modulation frequencies from the other terminal station, and selectively iiltering said modu lation frequencies yat each station to prevent the transmission of carrier waves modulated in either band of modulation frequencies towards the direction of their origin.

6. In a high frequency two way radio relay system, the combination of a rst and a second terminal station and one or more intermediate relay stations in which said first terminal station comprises a high frequency transmitter tuned to a iirst radio frequency, a source of modulation currents falling within a first band of modulation frequencies, means coupling said source to said transmitter, a high frequency receiver tuned to a second radio frequency, in which each intermediate relay station comprises a high frequency radio transmitter tuned to a frequency different from that of every other transmitter in the system within effective range, but equal to the resonant frequency of one receiver at each adjacent station, a pair of high frequency receivers, the first tuned to the frequency of the transmitter next adjacent in the direction of said first terminal station and the second tuned to the frequency of the transmitter next adjacent in the direction of said second terminal station, a first band pass filter passing said first band of modulation frequencies and coupled to the output of said first receiver, a second band. pass iter passing said second band of modulation frequencies coupled to the output of said second receiver, a mixing circuit to couple said transmitter to said first and second band pass iters whereby said trans mitter may be modulated by currents from said filters, and in which said second terminal station comprises a high frequency transmitter tuned to a frequency different from that of every other transmitter in the system` within effective range, but equal to the resonant frequency'of the receiver at the adjacent relay station which is coupled to the filter passing said second band of modulation frequencies, a high frequency receiver resonant to the frequency of the transmitter of the adjacent station, a source of modulation currents falling within said second band of modulation frequencies, and means coupling said source to said transmitter, whereby transmission is accomplished in one direction in a band of modulation frequencies distinct from that used for transmission in the other direction.

WARREN L. KNOTTS. HAROLD H. BEVERAGE. 

